Karangasem, Indonesia: A volcano with a deadly history continued to erupt Tuesday on Bali, one of the world’s most popular resort islands, spitting ash 4,000 metres high and stranding tens of thousands of tourists for a second day. Lava was welling in its crater, but it remained unclear how bad the eruption might get or how long it could last.

Authorities have raised the alert for Mount Agung to the highest level and told 100,000 people to leave an area extending 10 kilometres from its crater as it belches grey and white plumes into the sky. Its last major eruption in 1963 killed about 1,100 people.

Officials extended the closure of Bali’s international airport for another 24 hours due to concerns that jet engines could choke on the thick volcanic ash, which was moving across the island.

Tourists waiting for planes stared at information screens reading “cancelled” for every flight. Airport spokesman Ari Ahsanurrohim said more than 440 flights were cancelled Tuesday, affecting nearly 60,000 passengers, about the same as Monday. Without aircraft, getting in or out of Bali requires travelling hours by land and boat to an airport on another island.

“I don’t know, we can’t change it,” said stranded German traveler Gina Camp, who planned to go back outside and enjoy another day on the island, which attracts about 5 million visitors a year to its famed resorts and world-class surf spots. “It’s nature and we have to wait until it’s over.”

Experts said a larger, explosive eruption is possible or Agung could stay at its current level of activity for weeks.

“If it got much worse, it would be really hard to think of. You’ve got a huge population center, nearly a million people in Denpasar and surroundings, and it’s very difficult to envision moving those people further away,” said Richard Arculus, a volcano expert at Australian National University, adding that an eruption in 1843 was even more explosive than the one in 1963.

“There are many examples in history where you have this kind of seismic buildup—steam ejections of a little bit of ash, growing eruptions of ash to a full-scale stratosphere-reaching column of ash, which can presage a major volcanic event,” he said.

A Nasa satellite detected a thermal anomaly at the crater, said senior Indonesian volcanologist Gede Swantika. That means a pathway from the storage chamber in the volcano’s crust has opened, giving magma easier access to the surface.

Indonesian officials first raised the highest alert two months ago when a rash of seismic activity was detected at the mountain. More than 100,000 people living near the volcano fled their homes, many abandoning their livestock or selling them for a fraction of the normal price. The seismic activity decreased by the end of October, causing authorities to lower the alert level.

Tremors increased again last week and officials upped the alert and ordered another large-scale evacuation, with nearly 40,000 people now staying in 225 shelters, according to the Disaster Mitigation Agency in Karangasem. But tens of thousands of villagers have remained in their homes because they feel safe or don’t want to abandon their farms and livestock.

“Ash has covered my house on the floor, walls, banana trees outside, everywhere” said Wayan Lanus, who fled his village in Buana Giri with his wife and daughter.

Flows of volcanic mud have been spotted on Agung’s slopes, and Arculus warned more are possible since it’s the rainy season on Bali.

“They’re not making a lot of noise. It’s just suddenly coming like a flash flood out of nowhere,” he said. “You do not want to be near them. Stay out of the valleys.”

Indonesia sits on the Pacific “Ring of Fire” and has more than 120 active volcanoes.